Electric power assisted rack and pinion system

ABSTRACT

“An electrically operated power steering apparatus has an electric motor disposed parallel to a rack shaft. The system is designed such that the motor can be installed in any angle around a rack housing of smaller diameter. This permits easy installation and flexibility of design. The apparatus is also designed to accommodate off-the-shelf electric motors that are commercially available so as to eliminate the need to customize an electric motor for each application.”

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on, and claims the benefit of, U.S.Provisional Patent Application Nos. 60/159,796, filed on Oct. 15, 1999,and 60/151,829, filed on Sep. 1, 1999, the disclosures of both of whichare incorporated by reference herein in their entirety.

FIELD OF TECHNOLOGY

The present invention relates to an electric servo assist steeringsystem.

BACKGROUND OF THE INVENTION

It is known to electrically operate power steering apparatus forassisting the driver of a vehicle to turn a steering wheel.

In the known art of electric power assist, the steering torque of asteering shaft coupled to the steering wheel is detected and an electricmotor is energized based on the detected steering torque. The rotationalpower of the electric motor is transmitted through a ball-and-nutmechanism to the rack shaft to assist in turning the road wheels.

In the electrically operated power steering apparatus disclosed inJapanese laid-open patent publication No 59-50864, a ball-and-nutmechanism is positioned substantially centrally on a rack shaft, and therack shaft is radially supported at five locations including bushings.It is difficult to keep the rack shaft in full coaxial alignment at allof the five supporting locations. Because of alignment errors of therack shaft, different dimensional and assembling errors of the parts,and flexing displacements of the rack shaft caused by loads appliedthrough tires and tie rods, the ball-and-nut mechanism and arack-and-pinion mechanism are subject to undue forces tending todisplace or deform them or tend to suffer undesirable friction.

When this happens, the driver of the vehicle experiences poor steeringfeel and the efficiency of the electric motor of the power steeringapparatus is lowered.

An electric power assisted steering system is disclosed in Japaneselaid-open utility model publication No 59-172072, wherein a rack shaftis radially supported at two locations, i.e., a ball-and-nut mechanismand a rack-and-pinion mechanism. Because the rack shaft is radiallysupported at two positions and is maintained in better coaxial alignmentat those supporting positions, it is subjected to less undue forces.However, the rotor of an electric motor of the electrically operatedpower steering apparatus is supported in a cantilevered fashion due toits structural limitations. As a result, the distal end of the rotor isliable to interfere with an iron core that is fixed to rake housing.

In the electrically operated power steering apparatus disclosed in U.S.Pat. No. 5,650,701 an electric motor is disposed coaxially around therack shaft in a housing, the electric motor comprising a stator fixedlymounted in the housing and hollow rotor rotary supported in the housingby a pair of bearings mounted on respective opposite ends of the rotor,the ball-and-nut mechanism being coupled to the rotor, rack shaftaxially passing through motor rotor. This arrangement results in a largediameter housing that creates problem for installation in vehicles,particularly smaller vehicles where space is at a premium. Motorsintegrated into a housing have specific and tailored part designs thatrequire special production for very different car models therebyincreasing cost. Vehicle serviceability is also more expensive becauseany failure in the motor requires replacement of the entire assembly.Further, the motor shaft is directly connected to the ball nut. Motortorque is transmitted to axial movement of rack—thereby requiring alarge motor, large electric voltage for power, and larger dimensions ofelectric cable to generate the axial forces. Their design has largeinertial masses that have a negative effect on vehicle steering.

SUMMARY OF THE INVENTION

Disclosed is an electrically operated power steering apparatus that hasan electric motor disposed parallel to a rack shaft. The system isdesigned such that the motor can be installed in any angle around a rackhousing of small diameter. This permits easy installation andflexibility of design. The apparatus is also designed to accommodateoff-the-shelf electric motors that are already commercially available soas to eliminate the need to customize an electric motor for eachapplication.

The transfer of rotational energy from motor shaft to ball nut(secondary reducer) is realized by a primary reducer—a belt, silentchain, or gearing transmission. This primary reduction transmission willeffect a large torque on the ball-nut and a larger axial force on therack shaft that will satisfy the needs of even large vehicles. A smallermotor will be required, thereby also reducing the inertial mass on therack.

There is no need for large motors, high voltages or amperages, or largediameters of electric cable for large axial steering forces requested bylarge vehicle.

Less expensive vehicle serviceability is achieved because a defectivemotor may be replaced without having to replace the entire steeringapparatus.

A ball nut is installed between two angular contact ball bearings, whichpermit small angular rotations of the ball nut under steering forces.This reduces friction and insures smooth function of the ball-and-nutmechanism and the rack-and-pinion mechanism. Finally, the driver of thevehicle has a fine, smooth, quiet and comfortable steering feel, evendriving large vehicles.

The motor-shaft and rotor are not directly fixed on the ball nut andadaptive ball-nut movements therefore do not disturb motor function.

The rack and pinion will also preferably have a “V” shape and needlebearing, which has the effect of preventing rotation of the rack andreduces the friction associated with movement.

The housing may be covered by a strong one-piece ferrous cover. Thiscover rigidly holds the correct distance between two angular contactbearings even at high temperatures. Belt, gearing, or silent chaintransmission is installed inside the housing cover.

The steering wheel rotation is transmitted to the upper portion of thepinion shaft assembly. The pinion upper and lower shafts are connectedby a torsion bar. Relative rotation of the upper pinion shaft relativeto the lower pinion shaft is detected by torque sensor. This informationis transmitted to an Electronic Control Unit that powers the electricmotor. Rotation of electric motor is transmitted to axial movement ofrack shaft by belt transmission and ball-and-nut mechanism. Thissteering system uses energy only when it works, saving energy incomparison to traditional hydraulic power systems, which use hydraulicpumps that consume energy even when the vehicle is stationary.

The power steering system of this invention has a lower negative impacton environment relative to actual hydraulic power system because of thecleaner exhaust and lack of hydraulic fluids.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is outside view and cross-sectional view A—A of electric servoassist rack-and-pinion according to an embodiment of the presentinvention;

FIG. 2 is an transversal cross-sectional view E—E of rack-and-pinionmechanism of the electric servo assist apparatus shown in FIG. 1;

FIG. 3 is an transversal cross-sectional view B—B of rack-and-pinionmechanism of the electric servo assist apparatus shown in FIG. 1;

FIG. 4 is an transversal cross-sectional view D—D of rack-and-pinionmechanism of the electric servo assist apparatus shown in FIG. 1;

FIG. 5 is cross sectional view A—A of electric servo assistsrack-and-pinion according to an other embodiment of the presentinvention where torque reducer is released by gearing.

FIG. 6 is another embodiment of the invention having a v-shaped rack.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The disclosures herein are compatible with, and may be combined with,those of U.S. Provisional Application Serial No. 60/151,966 filed Sep.1, 1999; U.S. Provisional Application Serial No. 60/154,607 filed Sep.17, 1999; and U.S. Provisional Patent Applications having Serial Nos.09/650,869 and 09/633,549, both filed on Oct. 15, 1999; the disclosuresof all four of which are incorporated herein by reference in theirentirety.

As shown in FIG. 1 an electrical servo assisted rack-and-pinion withparallel motor-rack and ball screw, according to an embodiment of thepresent invention, has an elongate cylindrical housing 1, preferablycomposed of a single block of cast aluminum alloy. Rack housing 1 hasincorporated therein the housing of installation silent block 2 (FIG.2); pinion housing and housing of spring adjuster mechanism.

The electric servo assist rack-and-pinion also has a rack shaft 3inserted in the housing 1. Rack shaft 3 is coupled to respective tierods 4 that are connected to respective road wheels of a vehicle (notshown). When the rack shaft 3 moves axially (horizontally in thedrawing), it causes the tie rods 4 to turn road wheels to steer thevehicle. The range of travel is labeled with a lowercase “t” in FIGS. 1and 5.

Rotation of the vehicle steering wheel (not shown) causes rotation ofthe upper portion of the steering shaft 5, (FIG. 1 and FIG. 3) about itsown axis.

Rotation of the upper portion steering shaft 5 effects twisting oftorsion bar 6 (FIG. 3), causing relative rotation of the upper portionsteering shaft 5, to the lower portion of the steering shaft-pinion 7.Torque sensor 8 detects the relative rotation of the upper portionsteering shaft 5 to pinion 7 and sends a signal to the Electric ControlUnit 9. The Electric Control Unit 9 powers the electric motor 10.

Empowered electric motor 10 then rotates small pulley 11 that rotateslarge pulley 12 by belt 13. The ratio of large pulley and small pulleydiameter increases the motor torque on the large pulley 12. A relativelysmall electric motor 10 with a pulley ratio of about 3:1 can therebysatisfy the needed steering force on rack shaft 3 for most largevehicles. Of course, this pulley arrangement may be replaced with gearsfor greater precision and reliability, but at greater cost. A preferredcompromise would be a chain drive. The term “gear ratio” will beconstrued to encompass all such systems, whether gear or pulley.

The great advantage of this arrangement is that the motor may be placedjust about anywhere around the perimeter of the rack axis, depending onwhere space is needed for the particular design of the car. By utilizinga gear ratio, expensive high-torque motors may be avoided. Notice thatserviceability is greatly improved because the motor is easily removedand replaced, along with its gear or pulley. The motor is removablymounted for easy access and serviceability, so that the entire rack neednot be replaced when the motor or motor gear fails.

Large pulley 12 is affixed to ball nut 14 and transmits rotation to ballnut 14. Ball nut 14 is rotationally installed between two robust ballbearings 15 with angular contact.

Rotation of ball nut 14 is transformed into axial movement of the rackshaft 3 through a ball screw mechanism. Winding ball travel is providedon one side of rack shaft 3 while rack teeth are provided on the otherside of the rack shaft. The ball-and-nut mechanism is most preferablywithout backlash. Adjustment of the space in two bearings may beaccomplished by adjustment nut 16, which is locked by lock nut 17.

Ball bearing 15 is preferably designed to permit a small angularmovement of ball nut 14 around point “C” (FIG. 1). A preferred means ofaccomplishing this is disclosed in U.S. patent application Ser. No.60/154.607, the disclosures of which are incorporated herein in theirentirety. This small angular rotation of ball nut 14 will be under smallelastic deflection of rack shaft 3, avoiding large friction in theball-and-nut mechanism and insuring smooth, comfortably steering of thevehicle.

Ball nut 14, bearings 15 and large pulley 12 are disposed inside ballnut housing 18. It is preferred that the ball nut housing 18 stronglyholds constant the distance between the two angular contact bearings 15,even under high temperatures. It is preferred that this housing be madeof a ferrous metal or similarly strong material and heat-resistantmaterial. The ball nut 14 will preferably integrate the housing of asecond installed silent block 2 that transfers steering forces to thevehicle body.

On the other side of rack shaft 3, the teeth of pinion 7 are meshed withthe teeth of rack shaft 3. It is preferred that a spring adjustermechanism 19 (FIGS. 1, 3 and 5), or other equivalent apparatus, beprovided to eliminate any backlash between the rack 3 and the pinion 7.

As shown in FIG. 3, the housing 1 has a cylinder 21 positioned laterallyof the pinion 7. A rack bearing 20 is slideably disposed in the cylinder21 and is in sliding contact with the rack shaft 3 behind the pinion 7.The cylinder 21 has an open end remote from the rack 3 and closed by acap 22. The rack bearing 20 is normally biased by a spring 23 disposedunder compression between the rack bearing 20 and the Cap 22 in adirection to bias the rack shaft 3 against the pinion 7. The rack shaft3 is thus slideably supported by the rack bearing 20.

As illustrated in FIG. 1, the housing 1 has an axial end opening closedby a ball nut housing 18. Precise alignment between the housing 1 andball nut housing 18 is insured by bearing spacer 24.

In the electric servo assist rack-and-pinion, with parallel motor rack,and ball screw steering mechanism shown in FIGS. 1 through 4, the rackshaft is supported at two locations over a large distance. One supportis a rack bearing 20 that, under Spring 23, pushes rack shaft 3 towardpinion 7, thereby ensuring permanent contact between rack shaft 3 andpinion 7, even after wear of these two parts.

A second support is in the ball-and-nut mechanism that has ball nut 14mounted in bearings 15 with possibility of small angular self-alignmentwith first support. This self-alignment covers any new position of rack3 to pinion 7 and any elastic deformation of rack shaft 3 under steeringforces. These coaxial self alignments substantially reduce friction andincrease smooth steering.

Another advantage of the invention is that the ball-and-nut mechanism isinstalled in its own housing and does not disturb motor mechanism thatis separately rigidly fixed on to housings. Motor torque is increasinglytransferred to ball nut 14. The electric motor can be disassembled fromthe ball-and-screw mechanism and replaced with a new one withoutdisassembling the ball-and-screw or rack-and-pinion mechanisms.

FIG. 5 shows an Electric servo assist rack-and-pinion with parallelmotor-rack and ball-screw, according to another embodiment of thepresent invention. Those parts shown in FIG. 5 that are identical tothose shown in FIGS. 1 through 4 are denoted by identical referencenumerals, and will not be described in detail below.

As shown in FIG. 5, electric motor 10 is disposed closer to housing 1.Motor torque is transmitted to ball nut 14 by a gearing transmission.

Motor torque on ball nut 14 is increased by reduction of the gearingtransmission.

Primary smaller helical gear 26 is installed on motor shaft overtolerance rings 27. Secondary smaller helical gear 28 is installed overspline portion of primary smaller helical gear 26.

Back Lash spring 29 is installed between two smaller gears 26 and 28.Backlash spring pushes secondary smaller gear 28 away from primarysmaller gear 26, thereby effecting contact opposite the side having theteeth of secondary smaller gear 28 with large gear 30. This movementeliminates backlash between teeth of the smaller and larger gears.

Tolerance rings 27 permit small angular adjustment of the smaller gears26 and 28 to large gear 30. Tolerance rings 27 are calculated anddimensions determined in manner that they positively transfer motortorque, but slip under excessive torque, thereby protecting the gearteeth from overstress and breaking.

A silent chain transmission may be used instead of a gear or belttransmission.

Referring to FIG. 6, there is depicted the preferred V-shaped embodimentof the rack 3, having a V-shaped needle bearing 31. The needle bearing31 transfers force to a V-shaped guide 32, thereby significantlyreducing friction between the rack 3 and the guide 32. Resilient means23, such as a spring as shown will be used to bias the rack 3 againstthe pinion 7. A screw 22 and lock nut 34.

Although we have described what we consider to be the preferredembodiments of the invention, it will be understood that the inventionmay be embodied in other specific forms without departing from theessential characteristics and character thereof. The present embodimentsare therefore to considered in all respects as illustrative, and notrestrictive.

What is claimed is:
 1. An electric motor assisted steering apparatus fora vehicle, comprising: a rack having an axis defining an axial directionand a length thereof, said rack adapted to turning of a set of roadwheels, said rack supported at only two support locations along saidlength; a steering wheel and column, said steering wheel and columnadapted to move said rack back and forth along said axial direction soas to turn said road wheels; an electric motor; a screw mechanismadapted to convert rotational motion into axial movement of said racksaid screw mechanism is adapted to permit small angular movement arounda point coinciding with the axis of said rack and axially displaced atwhere said screw mechanism is located along said rack; said electricmotor removably mounted so as to be substantially easily replaceablewithout removal of said rack; said electric motor adapted to rotate saidscrew mechanism through a gear ratio; means for detecting a turningmotion applied to said steering wheel by an operator of the vehicle; anda controller for empowering said electric motor in response to saiddetected turning motion.
 2. The invention of claim 1 wherein said rackis V-shaped.
 3. The invention of claim 2 where said V-shaped rack is inoperable communication with said column via a pinion engaged with afirst side defining said v-shaped rack while an opposite second sidedefining said V-shaped rack is biased against said pinion with acomplimentary configured V-shaped needle bearing between resilent meandand said second side.
 4. The invention of claim 3 wherein needle bearingtransfers a force from said resilient means to said rack via a V-shapedguide significantly reducing friction between said rack and said guide.5. The invention of claim 3 wherein said resilient means includes aspring configured to bias said rack against said pinion.
 6. Theinvention of claim 3 wherein said resilient means allows deflection ofsaid rack.
 7. The invention of claim 3 wherein said only two supportlocations includes said screw mechanism and said needle bearing.
 8. Theinvention of claim 1 wherein said electric motor adapted to rotate saidscrew mechanism through said gear ratio includes belt transmissionbetween said electric motor and said screw mechanism.
 9. The inventionof claim 1 wherein said electric motor adapted to rotate said screwmechanism through said gear ratio includes gearing transmission betweensaid electric motor and said screw mechanism.
 10. The invention of claim1 wherein said electric motor adapted to rotate said screw mechanismthrough said gear ratio includes chain transmission between saidelectric motor and said screw mechanism.
 11. The invention of claim 1further comprising: a rack housing, within which said rack travels insaid axial direction; and wherein said electric motor is removablymounted to said rack housing so as to permit removal of said electricmotor without removal of said rack or rack housing.
 12. The invention ofclaim 1 further comprising: a rack housing, within which said racktravels in said axial direction; and wherein said electric motor isremovably mounted to said rack housing so as to permit removal of saidelectric motor without removal of said rack or rack housing.
 13. Theinvention of claim 1 wherein said adaptation for small angular movementfurther comprises: a ball nut mechanism; said ball nut mechanismmechanically interposed between said electric motor and said screwmechanism such that said electric motor rotates said ball nut mechanism,which in turn rotates said screw mechanism.